1. Field of the Invention
This invention is related generally to intracranial sensors for prevention of retractor blade injury (i.e., “retraction injury”) of the brain, and subdural monitoring devices.
2. Background
A retractor is an instrument used during surgery for, among other things, holding back structures adjacent to the immediate operative field (See, e.g., U.S. Pat. No. 5,769,781). During neurosurgical operations for aneurysms, tumors or other lesions located in the skull base, the surgeon must employ retracting devices in order to displace one or more lobes of the brain enough to gain adequate surgical exposure to the lesion. These retractors are adjusted by hand to optimize exposure. Unfortunately, it is very difficult for the surgeon to accurately gauge the amount of pressure actually applied to the brain during such placement of the retractor (see, e.g., Hongo et al., J Neurosurg 1987; 66:270–275). Moreover, it is also possible to inadvertently position the blade of the retractor such that a focal pressure point occurs at one particular area of the retractor blade pressing against the brain. Thus, injury to the brain can occur as a result of brain retraction when either the force applied is excessive or when the pressure is not adequately distributed to a large enough area of brain. This injury is thought to be the result of ischemia (inadequate blood flow) caused by the retraction, local trauma, or a combination of both.
It has been estimated that brain retraction injury occurs in approximately 10% of major cranial base tumor procedures and 5% of intracranial aneurysm surgeries (Andrews et al., Neurosurgery 1993; 33:1052–64). Various attempts have been made to develop technology to help minimize the incidence of this type of injury, with limited success. For example, a strain gauge or gauges attached to the retractor blade has been employed (Hongo et al., 1987; Rosenorn J., Acta Neurochir (Wein) 1987; 85:17–22). This approach has limited utility because pressure can only be measured from the point or points where the strain gauges are situated. As mentioned above, sometimes the brunt of the force occurs at the tip of the retractor blade where no strain gauge is present. Certainly this technique does little, if anything, to distribute force on the brain more evenly.
While knowing the amount of pressure applied will be helpful to the surgeon, the brain may be more or less sensitive to a given amount of pressure depending on its physiological state. The variables that influence the vulnerability of the brain to different degrees of retraction include the presence of subarachnoid hemorrhage (e.g., secondary to a ruptured aneurysm), depth of anesthesia, systemic parameters such as blood oxygen and carbon dioxide levels, and the particular region of the brain being retracted. As a result, electrophysiological monitoring of the brain can give a more accurate indication of when the threshold for injury is being approached. Intraoperative neurophysiological monitoring is commonplace during such operations, and typically the electroencephalogram (EEG) and somatosensory evoked potentials (SSEP) are employed. The electrodes commonly used are stainless steel. However, these modalities depend on the placement of electrodes on the scalp. Because of this, electrodes can only be placed to the extent that they do not interfere with the sterile surgical field, and obviously cannot be placed in the area of the craniotomy. Of course, this is precisely the part of the brain that needs to be monitored.
Thus, this type of recording from scalp leads can, at best, give information regarding regions of the brain adjacent to where the operation is occurring. Unfortunately, this method often reports erroneously favorable information that does not reflect the injury developing at the retractor site.
In neurosurgical operations where ischemia is anticipated (which includes most aneurysm procedures), high doses of anesthetics are typically administered to the patient to reduce cerebral metabolic rate and increase the tolerance of the brain to ischemia. Such high doses cause suppression of both the EEG and SSEP, thus rendering them ineffective for the detection of imminent brain injury. The recording of cortical direct current (DC) potentials from the brain is a technique that provides invaluable information about the functional status of the brain during situations of compromised blood flow (Ayad, doctoral dissertation ©1994; Sakaki et al., J Neurosurg 2001; 95:495–9). Further, DC potentials are not significantly affected by anesthetic agents. This technique was utilized experimentally in the 1950's during operations for epilepsy. However, with the exception of the clinical study currently investigating the device described herein, the benefits of DC potential for monitoring brain injury intraoperatively have not been put to use. This is, no doubt, because a practical method for applying the electrodes to relevant parts of the brain without obscuring the operative field has not been available.
Measuring such DC potentials requires the placement of a non-polarizable electrode (e.g., platinum or Ag—AgCl rather than stainless steel) on the cortical surface. Stainless steel and other electrodes cause artifactual potentials which prevent registration of the true voltage signal. The cortical electrode is referenced against a non-polarizable electrode placed at a remote site from the brain, and a high-input impedance DC amplifier is used to record the voltage. An extracerebral site is essential for the reference electrode in order for it to remain “indifferent” to injurious processes that may be occurring in the brain. Depolarizations (i.e., negative potentials) of greater than 4–5 mV typically develop when the brain is subjected to ischemia or trauma. DC potential is ideal for assessing the status of a small, localized area of the brain without penetrating its surface. Further, conventional EEG and SSEP recordings from platinum electrodes produce less noise than stainless steel electrodes during ischemia.
Strips of silicone rubber containing metal (e.g., Pt, stainless steel, etc.) electrodes known as “subdural grids” are commonly placed on the brain underneath the dura mater of certain patients with epilepsy for long-term recording of EEG to localize areas of seizure activity prior to surgery (e.g., U.S. Pat. No. 4,735,208). These materials are biocompatible, and have been tolerated very well for periods of up to several weeks when used for this purpose (Behrens E, Zentner J, et al. Subdural and depth electrodes in the presurgical evaluation of epilepsy. Acta Neurochir 1994; 128:84–87). However, such devices do not allow for monitoring of intracranial pressure. This latter capability would be helpful because many epileptics who have implanted grids develop local brain swelling, and recognizing a pressure increase would allow appropriate management of edema. Additionally, such a device, which also monitors intracranial pressure, may allow for concurrent measurement of such pressure and EEG postoperatively from severe head trauma patients who have undergone surgery for the management of said trauma.
Currently, there is no technology available to record electrical activity from areas of the brain that are being retracted. Instead, areas remote to the retraction site are monitored, providing sub-optimal and sometimes misleading information about the status of the brain. Two prior patents (U.S. Pat. Nos. 4,784,150 and 4,945,896) have incorporated technology to monitor local cerebral blood flow and metabolic parameters, respectively, into a brain retractor blade. Neither are equipped with a means for monitoring retraction pressure. Moreover, for the reasons discussed previously, local electrocortical activity provides a more readily interpretable index, compared to these measures, of when the threshold for injury is being reached.
With respect to monitoring retraction pressure, devices have been manufactured commercially in the past to perform this, however none are readily available at present (e.g., Codman CPM-100 Brain Retraction Pressure Monitor [Codman & Shurtleff, Inc. 15 Randolph, Mass.]; also see Hongo et al., 1987). Furthermore, such devices require proprietary accessory equipment that is expensive, and they are cumbersome to use (Andrews et al., 1993). Other devices are attached to the arm of the retractor blade rather than to the blade (McEwen et al., U.S. Pat. No. 5,201,325), and only convey information about the point of greatest pressure. Even where the “pressure responsive” surgical tool assembly is attached to the retractor blade (e.g. Nicholson, U.S. Pat. No. 4,263,900 and Lewis, U.S. Pat. No. 3,888,117), such devices do not permit recording of electrical activity.
The Codman CPM-100 device and the retraction pressure monitor of Nicholson (see also Donaghy et al., Am J Surg (1972) 123:429–31) all differ from the sensor of the present invention in other important structural respects. Retraction pressure monitors of Codman, Nicholson and Donaghy et al. employ an expandable reservoir with internal electrodes (i.e., the electrodes do not contact the underlying tissue). Air or other fluid is pumped into or out of the reservoir depending on electrical contact, resulting in inflation or deflation of the reservoir. Since air is one of the fluids utilized, there is no means for eliminating air from the system. Physiological saline cannot be used, as it would short-circuit the electrodes. Further, using air in these devices would not be compatible with conventional hydraulic pressure monitoring systems, and as such, any air-fluid interface would attenuate the fidelity and accuracy of the latter systems.
In contrast, the device described herein utilizes a flexible but relatively non-expandable bladder that can be, optionally, liquid-filled and free of air prior to use. This serves as an efficient mechanism to evenly distribute applied force throughout the entire area of contact. Alternatively, the device can be filled solely with air and free of liquid. The advantage of this type of system is that there is no hydraulic pressure exerted by a liquid column in the tubing which must be zeroed out during calibration (e.g. Bobo et al., U.S. Pat. No. 6,673,022). However, the disadvantage of employing air is that a somewhat expensive, specially designed measurement apparatus is required in order to measure the pressure within the bladder (e.g., Spiegelberg ICP monitor, [Aesculap, Inc., Center Valley, Pa., 2001]). The presently disclosed device provides an accurate means for measuring retraction pressure without the need for a fluid pump or internal electrodes.
In order to increase the tolerance of the brain to possible injury, various methods of neuroprotection may be employed during neurosurgical procedures such as the use of high doses of anesthetics, as discussed earlier. One of the most effective neuroprotectants is hypothermia, but there are limitations which prevent it from being used on a routine basis (see Connolly et al., Neurosurgery Clinics of North America (1998) 9(4):681–695). For example, generalized hypothermia to lower than 30° C. results in frequent cardiac arrhythmias and decreased blood clotting which may complicate surgery. Further, induction of deep hypothermia requires cardiopulmonary bypass, which is an involved procedure with risk of significant morbidity. On the other hand, local brain hypothermia may be equally effective as a neuroprotectant but does not suffer from the limitations of generalized hypothermia (see Ayad, doctoral dissertation ©1994). The device presently described provides a mechanism for induction of local brain hypothermia by instilling cooled liquid into the bladder via the double-lumen catheter.
There have been attempts to fabricate brain retractor blades which more favorably distribute pressure, so as to lessen the chance of injury (Vom Berg, U.S. Pat. No. 6,093,145 and Borsody, U.S. Pat. App. No. 2002/0022770). Neither are capable of monitoring retraction pressure or any other sensing modalities.
A second utility of the present invention pertains to its intracranial placement at the time of surgery for the purpose of postoperative monitoring of intracranial pressure (ICP), as well as the modalities of electrical activity described previously (i.e., as a subdural sensor). This information will permit the neurosurgeon and critical care physician to optimally manage brain swelling and injury after surgery.
There are two types of operations in which use of the subdural sensor (SS) may be indicated. The first was mentioned above, i.e., patients with intractable epilepsy that is refractory to anticonvulsant medication may undergo placement of many subdural electrode grids in order to localize the focus identified by the EEG monitoring. Because the placement of many grids involves some manipulation of surface of the brain, sometimes patients can develop significant brain swelling which results in abnormally raised ICP. Occasionally ICP is raised to the point that alteration of mental status occurs, and this may only be identified as such after the patient undergoes a CT scan of the head. By having a monitor of ICP in these patients, elevations in ICP can be identified sooner, and treated promptly with mannitol or steroids.
The second type of operation in which subdural placement of the device may be helpful is for severe closed head injury. In cases of cerebral contusion, massive swelling of the brain often occurs and if not treated appropriately, can result in coma or death of the patient. When severe swelling has occurred or is anticipated, often these patients are taken to the operating room to undergo removal of the damaged portions of the brain in order to provide room for the swollen brain and reduce ICP. Virtually all of these patients have some type of ICP monitor placed at the time of surgery to permit assessment of the swelling postoperatively. Currently there are two types of ICP monitors commonly used in this setting. The first is a ventriculostomy, which is an open-tipped catheter placed into the lateral ventricle of the brain and connected to a hydraulic transducer substantially similar to that used for the invention described. A ventriculostomy is advantageous because it permits not only measurement of pressure but also drainage of cerebrospinal fluid from the brain, which can aid in the lowering of ICP. However, after severe closed head injury, often the brain is so swollen that the ventricles are collapsed and placement of a ventriculostomy is impossible. The second type of ICP monitor currently available is an intraparenchymal probe which is placed into the substance of the brain through a small burr hole, and which records pressure from its tip by one of many methods (e.g., Camino fiberoptic monitor, Camino Laboratories, San Diego, Calif.). The disadvantages of this technique are that (1) it requires penetration of the brain with the probe, which itself causes a small amount of trauma, and (2) the pressure recorded from this type of probe is prone to non-trivial drift over a matter of days (Piper at al., Neurosurgery 2001; 49:1158–65). Furthermore, neither of these two types of ICP monitors permits the recording of local EEG or DC potential, which are valuable adjuncts in the assessment of brain injury. For these reasons, the present invention will be a superior monitoring device compared to the existing ICP monitors.
The tissue monitor described by Mayesvsky (U.S. Pat. No. 5,916,171) comprises a multiparametric apparatus able to monitor several modalities, including DC potential, ICP, a single channel of EEG, blood flow and NADH fluoremetry. Despite these aggregated modalities, there are a number of shortcomings. All parameters are recorded from the same, small area of cortex. Thus, all information will be reported from a small region that may not represent the bulk of the surrounding tissue, particularly, for example, if the area beneath the sensor is traumatized during placement, which might easily occur. Because it requires extensive, specialized equipment to operate, this system is clearly intended for use in a focused, research setting and not for routine monitoring of neurotrauma or epilepsy patients in the ICU.
As with a ventriculostomy, it should be noted that the pressure transducer utilized with the present device should be kept at the level of the distal end of the sensor (e.g., scalp incision) by the nursing staff in order to record an accurate pressure. Raising the transducer, due to the fluid column, will result in an artifactually low ICP and conversely, lowering the transducer below the appropriate level will cause an erroneously high ICP to be read. This aspect of the device can be readily managed by attentive staff, and is easily offset by the convenience, low cost, reliability and ubiquity of the standard hydraulic pressure monitoring apparatus in the intensive care unit and operating room. Alternatively, as discussed earlier, the sensor can be filled with air alone and not be susceptible to this fluid-column effect, but in that case would require the use of a specialized pressure measurement system (Speigelberg, 2001).
Last, various procedures require postoperative evacuation of residual fluids, for example, following craniotomy (see Jackson et al., Surgery (1971) 70:578–9). In fact drainage of serosanguineous wound fluid or CSF from the subdural space often plays an important part in the postoperative managemant of patients with craniotomies, especially trauma. Typically, the placement of drainage devices (e.g., Jackson-Pratt drain, Allegiance Healthcare, McGaw Park, Ill.) is similar to that for subdural grids. Consequently, many of the patients who would be candidates for placement of the subdural sensor would ordinarily have J-P drains placed at the time of surgery. Thus, a subdural sensor comprising a means to drain residual fluids would be a useful device.
The present invention as disclosed provides the desired capabilities absent in the foregoing devices, in that it is designed to permit monitoring of brain retraction pressure or ICP, as well as local cortical electrical activity, including DC potential (i.e., via multiple electrode sites). It permits registration of equilibrated pressure over the full length of contact of the retractor blade. Further, the present device as disclosed allows for redistribution of the forces applied to the brain during retraction so as to diminish the chance of focal brain injury during surgery. Protection of the brain from the rigid edges of the retractor blade is achieved without compromising visualization of the operative field. The induction of local brain hypothermia can be provided using the device for further neuroprotection during brain retraction. For use postoperatively, the instant device provides a closed system for egress of serosanguineous fluid into a sterile, external collector.